1. Field of the Invention
The present invention relates to a surface acoustic wave device and, particularly, to a surface acoustic wave device having a mounting structure with which a surface acoustic wave element (referred to as "chip", hereinafter) can be easily mounted on a surface of a support structure.
2. Description of Related Art
A conventional surface acoustic wave device is equipped with a chip which is a surface acoustic wave element comprising a piezoelectric substrate of such as LiNbO.sub.3. On one surface of the chip, electrodes constituting at least a pair of inter digital transducers are formed. The electrodes usually takes in the form of an interdigital electrode.
The chip is put in a recess of a surface mounting package of ceramics through an upper opening thereof with the surface of the chip on which the inter digital transducers are formed being upside. The opposite surface of the chip is die-bonded onto a bottom portion of the package by adhesive. Connecting pads of the inter digital transducers on the chip are connected to required portions by wire bonding. The upper opening of the package is seam-welded with a cap through a seam ring provided on a peripheral portion of the opening to hermetically seal the whole package. Surface wave generated by the interdigital electrode on the upper surface of the chip is reflected at end faces thereof in the surface wave propagating direction and the reflected surface wave portion interferes the main surface wave, resulting in degradation of its filtering characteristics. In order to prevent such interference from occurring, sound absorbing material such as silicon is provided at necessary portions of the end faces of the chip.
A construction of such surface acoustic wave device in which a chip is mounted on a metal stem having lead terminals and covered with a cap hermetically welded by resistance welding is also known. In this construction, an interior of the surface acoustic wave device is also hermetically sealed. In fixing the chip to the metal stem, the surface of the chip on which the interdigital electrode is formed is made upside. The other surface of the chip is die-bonded to a bottom surface of the metal stem by means of adhesive. Thereafter, input/output pad portions of the inter digital transducers are electrically connected to the lead terminals of the metal stem by wire-bonding.
The lead terminals are supported by holes of the metal stem filled with glass material and electrically insulated from the metal stem by the glass material. In this conventional construction of the surface acoustic wave device, the metal stem usually takes in the form of a substantially flat metal plate and the cap is substantially cup shaped such that it can cover the metal stem while allowing the chip, etc., to be mounted on an upper surface of the metal stem.
According to the conventional surface acoustic wave device mentioned above, a fabrication method therefor requires the steps of painting adhesive and hardening it for the die-bonding step and the wire-bonding step, etc. Further, the steps of painting the sound absorbing material and hardening the latter, etc., may be required additionally. Therefore, the fabrication of such surface acoustic wave device takes a long time and a large investment of facility is required. Further, in the conventional surface acoustic wave device, the structure itself is complicated and the size of the device may be increased due to the necessity of keeping a required attitude of the chip with respect to the metal stem during an assembling thereof and of proving electrical connections between various portions thereof.
Further, since, in order to facilitate a surface mounting of the device on a surface of a support structure, metallized pad portions must be provided on the package for the wire bonding, the package must have a space large enough for the metallized pad portions. In addition, in order to provide the workability of wire-bonding for the chip kept horizontally, it is necessary to provide a space having depth corresponding to a thickness of the chip within the package.
In order to respond to such requirements, the surface mount type package must be formed by, for example, using a triple-surface structure of same material such as ceramics, resulting in high cost.
Japanese Utility Model Application Laid-open No. S56-15118 discloses a surface acoustic wave device of this type in which a chip is die-bonded to a package substrate. In this construction, a cap is made from an insulating thin plate having a conductor pattern formed thereon and electrode terminal portions of the chip are fixed to the conductor pattern portion. In this device, the wire-bonding step may be removed. However, in order to remove the wire-bonding step after the die-bonding, the conductor pattern on the cap must be directly fixed to the electrode terminals of the chip. This construction is not suitable for the surface mount type device. That is, when such surface acoustic wave device as mentioned above is made as a surface mount type, the package must be hermetically sealed by the cap by such as seam-welding. However, since the cap is of insulating material, such welding is impossible. Therefore, in order to obtain a hermetic seal, at least outer peripheral portions of bonded portions must be molded by resin or low melting point glass material. Since such air-tight sealing processing tends to influence the reliability of connection between the chip and the conductor pattern, it is difficult to make such conventional device surface mount type.
Particularly, since the package, the chip and the insulating thin plate which are made of different materials having different thermal expansion coefficients, thermal expansion of, for example, the package may generate stress in the chip and in the electrical connections to the chip, causing the frequency characteristics of the surface acoustic wave device to be unstable.
Further, in any of the conventional surface acoustic wave devices mentioned above, the chip is mounted in the package with the inter digital transducers formed on the chip being upside. Therefore, impurity and/or dust originated in the package tends to adhere to the transducers directly, causing the characteristics of the device to shift and the electrical characteristics of the device to be degraded.
Particularly, in the surface acoustic wave device disclosed in Japanese Utility Model Application Laid-open No. S56-15118, the steps of painting adhesive and hardening it for the die-bonding step and the wire-bonding step, etc., are required. Therefore, the fabrication thereof requires a long time and large investment of facility. In addition thereto, since the chip is arranged on the metal stem with the inter digital transducers formed on the chip being up side, impurity originated in the package and dust from a plating material of the cap when the cap is mounted on the package and/or dust scattered together with discharge (arc) generated during resistance welding of the cap tend to adhere directly onto the inter digital transducers, causing the electrical characteristics to be degraded.
Further, since the metal stem and the chip of the above-mentioned conventional surface acoustic wave device are rigidly fixed to each other by the adhesive, deformation stress generated during the hermetic seal by means of resistance welding, etc., tends to be transmitted to the chip directly, so that, with change of deformation stress with time, filtering frequency tends to be changed.
Japanese Patent Application Laid-open No. H4-310009 discloses a surface acoustic wave device in which, in order to make a signal connection possible without using wire-bonding, signal input/output electrodes face to electrodes on a circuit board onto which the device is to be mounted and are capacitively coupled thereto. Although such construction is effective when a chip of the device is directly mounted on the circuit board, it is not suitable as a surface mount type device which is usually an independent part.
Japanese Patent Application Laid-open No. H4-170811 discloses a surface acoustic wave device in which the wire-bonding step is removed by forming metal bumps on bonding pad portions of a chip such that the bumps are in contact with metallized pad portions of a package. In this device, however, in order to obtain a reliable contact connection between the metal bumps formed on the chip and the metallized pad portions of the package, the bumps must be formed such that the flatness of a horizontal plane defined by these bumps becomes maximized. If the flatness is not high, there may be contact points at which electrical connection is not obtained, resulting in significantly defective characteristics. Further, even if the electrical characteristics of the package as a unit part is obtained, it may be subjected to a small deformation such as warping due to stress generated when the package is mounted on a printed circuit board or a ceramic substrate of some apparatus, which causes the contact connection to the metal bumps to be degraded, resulting in a degradation of characteristics due to the defective electrical connection.
Further, in the conventional surface acoustic wave device mentioned above, when the metal bumps are fixedly connected to the package by pressing with heat, the deformation stress when the package is mounted on the printed circuit board or the ceramic substrate and stress due to expansion and contraction of the package due to thermal stress under environmental condition may be transmitted to the connecting portions of the metal bumps. In order to prevent this problem, the contact area and contact strength of the metal bumps must be considered sufficiently.